It is previously known that a substantially vertical well bore can be turned with a short radius curved section into an inclined or horizontal well bore by providing a drilling tool which includes a bend section defining a transverse bend axis between a forward drill bit support portion and a trailing motor portion. The bend section of the drilling tool tends to steer the well bore so that it turns to a direction at right angles to a plane containing the bend axis. One particular example of this technique is disclosed in my U.S. Pat. No. 5,265,687. In this patent I also proposed that the bore be continued in a horizontal direction after the curved section is complete by adding shims to the underside of the drilling tool.
It is also known to steer a drilling tool during the drilling of a horizontal section by providing a drilling tool which has a slight angle, known as a fixed bent sub. This tool is then fed into the horizontal section and the whole tool rotated in the bore as the bit rotates. The speed of rotation of the tool, driven by the drill string, is slow relative to the rotation of the drill bit and can be of the order of 20 rpm relative to 200 rpm for the drill bit. This slow rotation of the drill string and bent sub has been found to keep the drilling direction more accurate than simply trying to guide a straight sub with shims or the like.
However, it is not possible to rotate the structure shown in my patent including the knuckle portion and the shims since this would put too much stress on the tool and lead to rapid mechanical break-down. Simply using a thinner shim and rotating the tool allows the tool too much movement and undue stress in the hole.
A number of different previous patents have shown shims or similar projections mounted on the side surface of the drilling tool to assist in guiding the drilling tool while the drill bit rotates. Examples of these are U.S. Pat. Nos. 4,492,276 (Kamp), 4,220,213 (Hamilton), 4,465,147 (Feenstra) and 4,442,908 (Steenbock). However as stated above, these shims or projections cannot allow the tool to rotate in the well bore at the slow rate of rotation to utilize the above technique for steering the drill bit.
U.S. Pat. No. 5,090,496 (Walker) discloses a fixed bent sub of the type mentioned above in which the tool housing is thickened on one side of the housing at the bend so as to attempt to increase the deviation of the drill bit from the straight line to provide a shorter radius of the curved section. The thickened portion of the housing is however entirely fixed to the housing and can not be removed. This prevents the tool from being used in the technique of my previous patent in which it is essential to have a bend section which allows bending of the tool from an initial coaxial position to a second position in which the knuckle joint between the two sections is offset to one side of the axis.
Various arrangements of eccentric collar are disclosed in U.S. Pat. Nos. 4,699,224 and 4,739,843 of Burton. Both of these patents disclose a drill string with a number of flexible or bend sections arranged in a row from the drill bit through to a straight section of the drill string arranged in the straight section of the bore hole. The eccentric collar is arranged at the bend section between the drill bit support portion and the next adjacent portion of the drill string. The collar includes a plurality of radially projecting elements or fins which lock the collar against the wall of the bore and thus prevent the collar from rotating. The drill string thus rotates within the collar and drives the drill bit. There is no possibility of the eccentric collar being locked to the drill string since the collar is intended for use during the drilling of the curved section and hence, if locked to the drill string would prevent the curvature from forming.